The article identifies the trends as well as documented instances of adversarial cyberattacks and hybrid warfare against NATO and EU Member States. It illustrates how these adversarial activities impact on the broader aspects of national security and defence, the socio-economic stability and the democratic order of the states affected, including on the cohesion of their respective societies. Cyberattacks by foreign actors—state and non-state—including state-sponsored attacks against democratic institutions, critical infrastructure and other governmental, military, economic, academic, social and corporate systems of both EU and NATO Member States have noticeably multiplied and have become more sophisticated, more destructive, more expensive and often indiscriminate. The cyber and hybrid threats are increasingly seen as a strategic challenge. The article presents some salient topics such as the nexus between cyberattacks and hybrid operations; the game-changing artificial intelligence dimension of the cyber threat; and the viability of public attributions in cases of cyberattacks. On the basis of analysis of the conceptual thinking and policy guidelines of NATO, the European Union and of the U.S., the author makes the case that a resolute Trans-Atlantic cooperation in the cyber domain is good for the security of the countries involved and essential for the stability of today’s cyber-reliant world.